The determination of inorganic constituents in concentrated acids and bases is important in a variety of chemical and other processes. However, when using ion chromatography (IC) it is often difficult to detect trace amounts of anions in concentrated bases or trace amounts of cations in concentrated acids. In other words, anion samples that are highly basic (high concentration of hydroxide) or cation samples that are highly acidic (high concentration of hydronium ions) are difficult to analyze by IC because the high concentration of hydroxide or hydronium ions mask the ion peaks (either anion or cation, respectively) of interest.
In order to address this problem, one method used in the art is to pretreat the sample using an ion resin or ion exchange bed to remove the interfering ions (e.g., the hydroxide ions in anion analysis and hydronium ions in cation analysis) from the sample. Accordingly, the interfering hydroxide or hydroniun ions may be removed by passing the sample through an ion-exchange bed or ion exchange resin which removes the interfering ions according to the following neutralization reaction:
For anion analysis, the sample is passed through a cation exchange resin in the hydronium form (X=sample anion, M=sample/hydroxide countercation): EQU MOH(sample)+Resin-H.fwdarw.Resin-M+H2O
Excess sample hydroxide is neutralized to water. Sample anions are converted to their corresponding acids: EQU MX(sample)+Resin-H.fwdarw.Resin-M+HX
For cation analysis, the sample is passed through an anion-exchange resin in the hydroxide form (X=sample/acid counteranion, M=sample cation). EQU HX(sample)+Resin-OH.fwdarw.Resin-X+H2O
Excess sample acid is neutralized to water. Sample cations are converted to their corresponding hydroxide salts: EQU MX(sample)+Resin-OH.fwdarw.Resin-X+MOH
One prior art method for accomplishing the above described neutralization of interfering ions is passing the sample through a disposable pre-treatment, ion-exchange bed before flowing the sample ions to the analytical column. One such disposable ion-exchange bed known in the art is sold by the assignee of this application, ALLTECH ASSOCIATES, Inc. This pre-treatment neutralization column is sold by ALLTECH under the name ALLTECH's Maxi-Clean.TM. IC-OH or IC-H cartridges. These ALLTECH devices are solid-phase extraction devices used to eliminate interfering ions (e.g. matrix interferences) from samples prior to analysis by ion chromatography. The foregoing ALLTECH devices, as well as the other similar neutralization columns or cartridges presently on the market, suffer from the following drawbacks:
1) Each cartridge is normally used only once and then discarded. This can be expensive. PA1 2) Passing the samples through the cartridges is a manual, labor intensive process that can be difficult to automate.
There have been attempts by others to address the problems of the disposable neutralization columns described above. One such method is disclosed by Siriraks and Stillian (Journal of Chromatography, 640 (1993) 151-160). Sirirak et al. disclose an electrolytically regenerated micromembrane-based technique for removing matrix interferences and neutralizing samples. According to this technique, a self-regenerating suppressor (SRS) device, which is a suppressor for reducing background noise of the mobile phase after the analytical column, is also disclosed for use as a pre-treatment (e.g., before flowing the sample to the analytical column) device for neutralization of the sample. The SRS devise is thus disclosed as a pretreatment device for trace anion determination in concentrated bases and trace cation analysis in concentrated acids.
The technique disclosed by Sirirak et al. avoids some of the prior art problems such as, for example, the need for disposing of the pretreatment cartridge after every run. The SRS device is self-regenerating. For a more detailed discussion of the SRS system, those skilled in the art are referred to the above cited Sirirak et al. article.
Despite these improvements, the SRS device is, however, not without its shortcomings. The SRS device uses sensitive membranes. These membranes have inherently low ion-exchange capacity, compared to ion-exchange resin beds, and require a complex recycling/monitoring scheme to completely neutralize strongly acidic or basic samples. Additionally, membrane-based suppressors are inherently fragile and are susceptible to rupturing under the high-pressures present ahead of the column in an IC system. Consequently, additional valves are required to neutralize the sample off-line and then insert it into the analysis stream. A separate stream of high-purity water is also required to feed the electrolytic micromembrane suppressor (the SRS) during regeneration, adding further expense and complication to the device. The present invention is intended to address the foregoing problems in the art relating to sample neutralization.
Another pre-treatment method known in the art is pre-concentrating relatively dilute samples for better detection and quantification. Many samples contain trace amounts of anions and cations at levels too low to detect by direct injection into an IC system, even where interfering ions (hydroxide and hydronium) are not present. In these situations, samples are normally pumped onto a short ion-exchange column (the pre-concentration column), which traps the sample ions of interest while the balance of the sample is flowed to waste. The trapped sample ions are then eluted from the pre-concentration column in a much smaller volume and, thus, at a correspondingly much greater concentration than in the original sample. The highly concentrated sample is then flowed to an analytical column for separation and then to a detector for detection and quantification.
In prior art pre-concentration systems, the pre-concentration column is usually installed onto a six-port sample injection valve and the sample is delivered to the preconcentration column by a separate pump. Thus, this system requires an additional pump in the IC system and can also be difficult to automate.
The present invention is also intended to address these problems as well.